Processes and media for quenching metals



United States Patent 3,281,288 PROCESSES AND MEDIA FOR QUENCHING METALS Arthur James Carver, Wallingford, Benjamin Thomas Fowler, Abingdon, and Aubrey Turner Langton, Milton, England, assignors to Esso Research and Engineering Company, a corporation of Delaware No Drawing. Filed Nov. 27, 1963, Ser. No. 326,397

14 Claims. (Clr 14820.6)

The present invention relates to processes for quenching hot metals and is particularly concerned with the liquid compositions used as quenching media in. such processes. In one particular aspect, this invention is concerned with the quenching of steel, especially to a temperature' in the range in which the martensitic structure is stabilized. Such a quenching process is known in the art as mar-quenching. Many metal alloys, particularly ferrous alloys and carbon steels require heat treatment to develop desired degrees of hardness and strength. These properties depend to a large extent on the physical structure of the metal in question. For example, in the case of steel, a martensitic structure is desirable since it is a hard structure which is particularly suited to gear applications, etc. To increase the hardness of metal. alloys, they areheated to an elevated temperature, e.g. above 1600 F., and then plunged into a comparatively cool quenching medium. Sometimes, the desired structure may be obtained immediately. Other times, the metals are quenched to maximum hardness and then tempered to achieve the desired hardness and ductility. I

Numerous quenching media are known in the art and they are based to a great extent on petroleum hydrocarbon oils to which small amounts, e.g. 0.1 to wt. percent, of various chemicals have been added, which chemicals accelerate the speed of quenching. Alternatively, water-based quenching media have been proposed. See for example US. 2,485,103, U.S. 2,600,290, and US. 2,857,301. A careful study of the pertinent art will reveal that those routinely skilled in the art have continuously sought to develop quenching media along these lines (petroleum or water-based), which media would give effective quenching at a low initial unit cost. Consequently, those skilled in the art have imposed limitations (i.e. quenching media must be water or petroleum based) on their quests for new media, which limitations the present inventors have now found to be unnecessary.v

It has been discovered, and this discovery forms the basis of the present invention, that synthetic esters such as are used as high temperature lubricants (e.g. turbolubricants) or as plasticizers for synthetic resins or polymers, have properties which cause them to be particularly useful as quenching media. These synthetic esters (preferably synthetic carboxylic acid esters) are superior, in many respects, to petroleum or water-based media for many quenching operations. Additionally, these synthetic esters are more heat-stable than the usual hydrocarbon oils. Because less thermal decomposition takes place and because these esters preferably have a lower viscosity (thus creating less carry-off of oil by the metal) their long range cost has been demonstrated in plant-scale experimental tests to be similar to that of the hydrocarbon oils which require more frequent replacement and more make-up. An important advantage of these esters is that replacement does occur less often. Thus, factories can be operated more efficiently in other areas, e.g. milling, grinding, etc., due to the fact that the quenching operation is no longer a bottleneck. This feature can result in t'remendous savings.

By way of example, petroleum hydrocarbon oils are frequently used in mar-quenching, but such oils are un- 3,281,288 Patented Oct. 25, 1966 pleasant to use at bulk temperatures above about 130 C. which is well below the martensitic transition temperature (usually in the range of 200 to 220 C.). Consequently, one common practice is to use these hydrocarbon oils at a bulk temperature of about 130 C. and to quench the steel for a strictly limited period of time to avoid cooling a significant surface layer of the steel below the martensitic transition temperature. Obviously, it would be clearly advantageous to use a quenching liquid at a bulk temperature at or slightly above the martensitic transition point so that the critically limited quenching period could be avoided. The present inventors have dis- I covered that this can be accomplished by using synthetic esters. v 7

Synthetic esters, e.g. pentaerythritol esters, were observed by the inventors to have more attractive high temperature characteristics, and in particular, a lower viscosity.and a lower volatility than the hydrocarbon oils.

Moreover, many of these esters are still liquid at temperatures of 200 or above which is desirable for marquenching. Whenused for mar-quenching, these esters, e.g.v .pentaerythritol esters, have been shown by the inventors to be quite effective. As a result of their lower viscosity,- less carry-off of oil by the quenched metal occurs and less make-up of quenching oil is necessary. For example, a make-up of 11 liters per ton of quenched steel parts has been observed to be required at a bulk oil temperature of 130 C. if an oil having a viscosity of 16 centistokes at 210F. is used, whereas only about 1 liter per ton isnecessary at a bulk oil temperature of 210 C.

i if an-oil having. a viscosity of 5 centistokes at 210 F. is

used. These viscosities at 210 F. are respectively typical of petroleum hydrocarbon. quenching oils currently used at bulk oil temperatures of about 130 C. and of the inventive synthetic ester oils used at bulk oil temperatures of 200 to 220 C.-

The syntheticester oils which may be advantageously used according to the present invention include the varicompositions.

ous monoesters, diesters, polyesters, complex esters, etc., as well as mixtures thereof. According to the present invention, the quenching medium employed will comprise a major proportion of. synthetic ester, which ester is preferably of the carboxylic acid ester type. The remaining portion of the quenching media can be various mineral oils, polyphenyl ethers, etc., as well as conventional additives. This does not mean to imply, however, that all such esters or combinations of esters and/or other fluids serve with equal efliciency. It is preferred that the quenching medium employed consist essentially of carboxylic acid esters or mixtures thereof. Thus, it is preferred to exclude significant amounts of mineral oils or other fluids (e.g., polyphenylethers) from the present Of course, it is possible to employ varying amounts of mineral oilin admixture with the preferred synthetic esters and thereby achieve results which embody the present inventive concept, but which do not possess all of the superior properties which may be obtainedby employing only the carboxylic acid esters or mixtures thereof when used in conjunction with certain additives (e.g., antioxidants). When, for some reason, e.g., economics, it becomes desirable to employ lubricating oils of the petroleum hydrocarbon type, it will generally be preferred that these oils have a viscosity of from-40 to 150, e.g., 60 to 1 15 S.S.U. at F. These oils may be paraffinic, naphthenic, or aromatic in nature.

Aromatic oils may be preferred for some applications.

propanediol-lfi) and monohydric alcohols methylcyclohexylmethanol or 2,2,4-trimethylpentanol).

' tages of no-beta hydrogen esters.

206, 2,723,286, 2,705,724, 2,743,234, and 2,575,196.

These references are incorporated herein in their entirety. These ester oils are generally characterized by viscosity properties that are outstanding at both high and low temperatures, and in this respect are quite superior to mineral oils. Because of their utility over extremely wide temperature ranges, the synthetic ester lubricating oils are widely used in the formulation of lubricants for aircraft engines such as turbo-jet, turbo-prop, purejet, and turbo-fan engines.

The carboxylic monoesters are formed by reacting monobasic acids (e.g., C -C acids) With monohydric alcohols (e.g., C C alcohols). The monoesters are the least preferred because of volatility and viscosity considerations. However, they may often be used to advantage in combination with other synthetic esters, e.g., complex esters as described in US. 2,499,984.

The carboxylic diesters are formed by reacting dibasic acids (e.g., sebacic acid) with monohydric alcohols (e.g., 2-ethylhexanol) or by reacting glycols (e.g., propylene glycol) with monobasic acids (e.g., n-hexanoic acid). These diesters usually contain from 16 to 36 carbon atoms, e.g., 22 to 26 carbon atoms.

The poly esters are formed, for example, by reacting polyhydroxy compounds (e.g., trimethylolpropane or pentaerythritol) with straight or branched chained monocarboxylic acids (e.g., lauric acid). Alternately, polybasic acids (e.g., tricarballylic acid) may be reacted with monohydric alcohols (e.g., ethanol).

The complex esters are formed by reacting dicarboxylic acids (e.g., adipic acid) with glycols (e.g., 2,2-dimethyl In general these complex esters will have a total of 20 to 80, e.g., 40 to 65, carbon atoms per molecule. Each molecule will be of the general formula:

alcohol (acid-glycol) acid-alcohol wherein x will be from 1 to 6 and usually average about 1.8. In general, 0.3 to 2.4 moles of alcohol and 1 to 2 moles of acid can be reacted per mole of glycol. By reacting 1 mole of glycols, 2 moles of acid and 2 moles of alcohol, a mixture will result which may comprise about 35 wt. percent of the diester of the acid and alcohol (i.e., x will be zero) and about 65 wt. percent of the complex ester (i.e., x is from 1-6). By varying the amount of reactants, varying ratios of diester to complex ester can be obtained. Alternatively, acid-centered complex esters may be prepared and monocarboxylic acids used 7 to terminate the chain.

' compounds, have been found to be more stable at high temperatures when compared with conventional diesters having beta hydrogen. To obtain even more thermal stability, esters may be formed from no-beta hydrogen alcohols and/or no-beta hydrogen polyols and no-alpha hydrogen acids including aromatic carboxylic acids. These esters are thought to have an even higher degree of thermal stability without some of the usual disadvan- Such esters, by virtue of their structure, are also of particularly low volatility and high flash point and have low viscosities at temperatures of 200 to 220 C. However, these materials have not yet found widespread utility because of their high initial cost and manufacturing difficulties.

Representative compounds that may be used in preparing the synthetic esters (both conventional and hindered) for use in the present invention are:

Mor ocarboxylieacids.n-Butanoic acid; 2-ethyl butyric acid; caproic acid; heptanoic acid; caprylic acid; Z-ethyl stability).

4 hexanoic acid; pelargonic acid; capric acid; lauric acid; C Oxo acids; C Oxo acids; stearic acid; at,a-dimethyl valeric acid; a,a-dimethyl capric acid; margaric acid; oleic acid; arachidic acid; etc. I

Dz'carboxylic acids.-Sebacic acid; a-dipic acid; azelaic acid; d'odecanedioic acid; pimelic acid; succinic acid; isosebacic acid; thioglycolic acid; dioleic acid; phthalic acids;

' octylene glycol; 2 ethoxy-Z,4-di-methylpentane-1,5-diol; trimethylol propane monoesters or monoethers; dimethylol benzenes; etc.

Polyhydroxy compounds-Trimethylol ethane; trimethylol propane; trimethylol 'heptane; trimethylolisooctane; trimethylol decane; pentaerythritol; dipentaerythritol; ditrimethylolpropane; triand tetramethylol benzenes; etc. 1

These various monoesters, diesters, triesters, complex esters, etc., may be combined in almost any fashion to effectively tailor an oil to a particular need. Simple mixing is usually sufiicient. For example, it is known to com- :bine trimethylolpropane esters with pentaerythritol esters; either or both of them with diesters or complex esters (both with and without beta hydrogen or alpha hydrogen), etc.

A comprehensive textual treatment of synthetic lubricants is available. It is Synthetic Lubricants, edited by R. C. Gunderson and A. W. Hart (Reinhold Publishing Corp., 1962). Chapters 5 and 1-0 are particularly appropriate. They deal with dibasic acid esters and neopentylpolyol esters, respectively, which esters are useful according to the present invention. That text is incorporated herein :by express reference. Among all of the possible esters included in this seemmgly infinite variety of synthetic esters, it has been found that one particular class of esters, especially when used n combination with various additives (e.g. antioxidants) is most outstanding. These esters are the neopentyl polyol esters, especially the full esters of trimethylol propane and of pentaerythrit-ol. These preferred esters may be used alone, mixed with each other, or mixed with the previously described diesters, complex esters, etc., especially those other esters containing no beta hydrogen. When used alone or mixed with each other, they are good mar-quenching media.

Although numerous acids may be used to esterify trimethylol propane, generally it is advantageous to employ the C to C monocarboxylic acids. Particularly preferred are the C to C e.g. C to C acids including those having no alpha hydrogen (for improved thermal Some of the lower molecular Weight esters (e.g. 07-019 acid esters) of trimethylol propane are commercially available from such sources as Emery Industries, Cincinnati, Ohio. These commercial esters are effective for use in the inventive procms, although their flash properties, etc. are not as desirable for mar-quenching as are the longer chain, e.g. C -C acid esters. Consequently, the longer chain acid esters of trimethylol propane and the other neopentyl polyols, e.g. pentaerythritol, are preferred, especially for mar-quenching where" higher fluid Particularly preferred are esters of this type which are formed from acids having a chain length of at least 11 carbon atoms and preferably a chain length of from about C to C Mixtures of acids may be used, which mixtures of acids will have an average chain length Within the recited ranges.

Of the neopentylpolyol esters, the tetra esters of pentaerythritol are the most preferred. The acids which can be used to esterify pentaerythritol include the C to C e.g. C to C monocarboxylic acids. Particularly preferred for mar-quenching are those pentaerythritol tetra esters of monocarboxylic acids having a chain length of at least 11 carbon atoms, and preferably a chain length of from 14 to 18 carbon atoms. Alternatively, a mixture of monocarboxylic acids having an average chain length of such numbers of carbon atoms may be used.

According to another aspect of the invention, the quenching media will consist essentially of a tetra-ester of a tetrahydric alcohol (e.g. pentaerythritol) and a monocarboxylic acid having a chain length of at least 5 carbon atoms and substituted at the carbon atom in the alphaposition to the 'carboxylic acid group, with. two alkyl groups. Alternatively, a mixture of monocarboxylic acids having an average chain length of at least 5 carbon atoms, each being substituted at the carbon atom in the alpha-position to the carboxylic group, with two alkyl groups may be used. Preferably at. least one of the alkyl groups attached to the alpha carbon atom is a methyl or ethyl group, and preferably the carboxylic acid contains a total of at least 9, but not more than 20, carbon atoms in its molecule, or the mixture of carboxylic acids contains an average of at least 9, but not more than 20, carbon atoms per molecule.

Examples of monocarboxylic acids having a chain length of at least 11 carbon atoms are lauric acid (C myristic acid (C palmitic acid (C margaric acid (C stearic and oleic acids (C and arachidic acid (C Mixtures of acids having an average chain length of at least 11 carbon atoms include those derived from the hydrolysis of animal fats and vegetable oils, such as tall oil, palm oil and coconut oil.

Monocarboxylic acids having a chain length of at least 5 carbon atoms and substituted at the alpha carbon atom by two alkyl groups are members of the class of acids known as neo-a-cids, and may also be defined as trialkyl acetic acids in which at least one of the alkyl groups has a chain length of at least 3 carbon atoms. Such acids can be synthesized from suitable olefins by reaction with carbon monoxide in the presence of an acid catalyst and hydrolysis of the reaction product. Examples of such acids are: 1,1-dimethyl valeric acid (dimethyl propyl acetic acid); 1,1,3-trimethyl heptanoic acid (synthesized from propylene trimer); and other C and C acids synthesized from polymerized propylene, buty-lene or isobutylene fractions containing C and C olefins, respectively.

One particular synthetic ester which can be used as a quenching medium is a tetra-ester of pentaerythritol and a mixture of monocarboxylic acids having an average chain length of between 6 and 10 carbon atoms, known as Hercoflex 600. This ester is commercially available from Hercules Powder Co., Wilmington, Delaware. These shorter chain acid esters are not as well suited for marquenching as are the longer chain (e.g. C plus) acid esters.

The flash points of the synthetic ester quenching oils of the present invention are generally much higher than those of the hydrocarbon quenching oils used at the present time. The flash point of a tetra-ester of pentaerythritol and a mixture of monocarboxylic acids having an average chain length of 12 to carbon atoms, for example, is about 295 C., whereas that of a typical commercial hydrocarbon quenching oil is 243 C. The flash point of Hercoflex 600, an ester of pentaerythritol and a mixture of monocarboxylic acids having an average chain length of between 6 and 10 carbon atoms, is about 260 C. Clearly the preferred esters of the present invention will be much safer to use at bulk quenching oil temperatures of 200 C. or above than hydrocarbon quenching oils. Also they have a lower viscosity at the bulk oil temperature and so require less make-up, as hereinbefore described. Also, they are less volatile at the bulk oil temperature than hydrocarbon oils, thus leading to lower evaporation losses.

Preferably the synthetic carboxylic acid ester quenching media used according to the present invention will have a kinematic viscosity at 210 F. of below 10 centistokes. Preferably the viscosity will be below about 8.5 centistokes. cosity at 210 F. will be :below 7" centistokes e.g. 5 cs. The viscosity at the bulk oil temperatures used for marquenching, (e.g. 210 C.) will preferably be below 5 centistokes, more preferably below about 2.0 centistokes, e.g. below 1.0 centistoke. Hercoflex 600 has a kinematic viscosity at 210 F. of 4.6 centistokes and at 210 C. only about 0.8 centistoke.

The state of the present lubricants artis such that selection of suitable esters and combinations thereof, together with selection of the necesary additives, can be done with ease by the routineer. For example, a current military specification for aircraft turbo lubricants (Mil-L-7808) encompasses an oil having suitable qualities for general use according to the present invention. Similarly, Pratt & Whitney Specification 521-B for turbofan lubricants will also produce an oil which is suited for use according to the present invention. No known mineral oils of the petroleum hydrocarbon type are now able to meet the stringent requirements as set up, for example, by these two specifications. Synthetic carboxylic acid esters, however, are currently employed by lubricant manufacturers in meeting these rigid requirements. Obviously, however, a number of requirements set up by these specifications are not critical when the ester oils are to be used as quenching media, e.g. Ryder Gear Load, etc. However, their viscosity requirements and oxidation stability requirements are especially desirable.

The use of additives to improve specific performance properties of synthetic esters for particular applications is commonplace in the synthetic lubricants art. These same additives may be used to greater or lesser extent when employing theseester oils as quenching media. The use of antioxidants is preferred, especially for marquenching operations. Any conventional antioxidant may be employed, e.g. phenyl, alpha naphthylamine or phenyl beta naphthylamine. It is preferred to use a mixture of antioxidants consisting essentially of phenothiazine, or its alkyl or alkoxy derivatives and a secondary amine, e.g. dioctyl diphenylamine. Where extremely severe conditions are employed, it is preferred to use phenothiazine, per se, in combination with a secondary amine, preferably one having 2 benzene rings attached to the nitrogen atom. It is preferred that not less than about 0.3% and not more than 2% by weight (based on the total weight of the ester portion of the composition) of phenothiazine be employed. The weight ratio of secondary amine to phenothiazine should be from 2:1 to 10:1, e.g. 3:1 to 8:1. A particularly desirable combination of these additives is from 0.3 to 2 wt. percent (e.g. 0.5 to 1.0 Wt. percent) phenothiazine in combination with 3 to 6 wt. percent (e.g. 4 to 6 wt. percent) of the secondary aromatic amine.

When using, for example, tetraesters containing the preferred antioxidant combination, it has been found that they have improved oxidation resistance at high temperatures when compared with conventional hydrocarbon quenching oils and when compared with the same tetraesters containing 1 wt. percent phenyl alpha-naphthylamine.

The preferred secondary amine is dioctyl diphenyl amine. These amines are commercially available and are sold under such trade names as Vanlube 81, Agerite Stalite, Agerite Stalite S, Octamine, etc.

Even more preferable, the minimum visclude a preferred embodiment. cated, all parts are by weight and all percentages are 7. The alkyl and alkoxy derivatives of both phenothiazine and diphenylamine will usually contain from 1 to 20 carbon atoms per alkyl or alkoxy group. Preferably, these groups will each contain from 4 to 12 carbon atoms, e. g.

6 to 10 carbon atoms. The dialkyl and dialkoxy derivatives of diphenyl amine are preferred. Similar di-substituted derivatives of phenothiazine may be used, although phenothiazine, per se, is preferred.

The actual selection of additives and the amounts employed are believed to be well within the skill of those routinely engaged in the lubricating art particularly in view of the present disclosure. Any antioxidant for lubricating oil use may be employed although some are more preferred than others. See for example Australian patent specification 236,748 and British patent specification 921,238. Other additives may be employed if desired, e.g. rust inhibitors, corrosion inhibitors, non-foaming agents, dyes, thickeners, etc. The total amount of additivesemployed will usually be below 20 wt. per-v cent based on the total weight of the base oil portion of the quenching media and more frequently will range from 0.5 to 10 wt. percent.

The present invention will be more clearly understood by reference to the following specific examples which in- Unless otherwise indiweight percentages. I

EXAMPLE 1 To demonstrate the efiectiveness of synthetic carboxylic esters, alone or in combination, as quenching media,

the following method may be followed:

Then, the quenched specimen may be suitable prepared, e.g. sectioned, mounted in plastic, polished, etched, and observed under a microscope (e.g. photomicroscope) to determine whether or not the desired metallurgical changes have occurred. In this respect they may be compared with test specimens similarly prepared, but using instead a commercially available mineral oil quench medium.

Quenching speeds may be determined using a G.M. magnetic quenchometer (an instrument and test method developed by General Motors Corp., Detroit, Michigan). The quenchometer and test method are fully described in a brochure published by General Motors Corp. in October 1957, entitled Process DevelopmentsMagnetic Quench Test.

Briefly, the test makes use of the properties of metals which lose their magnetism when heated above a certain temperature known as the Curie point and regain their magnetism when cooled below this temperature. High purity nickel with a Curie point of about 670 F. is normally used as the metal because of its non-scaling characteristics and resistance to cracking upon repeated heating and cooling. A spherical nickel specimen having a diameter of '%3 inch and weighing about 50 grams is used in cooperation with 200 cc. of quench medium to be tested. The nickel sphere is heated to 1625 F. and then plunged into the quench medium being tested. The quench medium is acted on by a magnetic field. The time required (as measured electrically) for the ball to be drawn to the magnet after immersion is a measure of the quenching speed of the quench medium being tested.

When nineteen commercially available quenching oils are evaluated by this procedure, the average quench speed is observed to be about 2223 seconds with the fastest speed being 12.5-13.5 seconds. The slowest speed noted is about 3031 seconds.

The synthetic ester quench oils of the present invention compare well with these conventional prior art oils when they are tested under the same conditions, their cooling capacities being comparable with those of the conventional oils. Moreover, as previously indicated, they are know to be much more thermally stable and consequently require less frequent replacement. Because of their generally lower viscosity at operating temperature, they also require less make-up due to carry-oil. While this lower viscosity is a desirable and advantageous feature, it is not critical. Moreover, the preferred neopentyl polyol esters, e.g. pentaerythritol tetraesters, are

much more effective than conventional oil-based quenching media for mar-quenching because they can be used at higher bulk temperatures (eg. 200 C.) and thus obviate the need for critically limited quenching times. Obviously, their use at lower temperatures will be equally as effective for ordinary quenching.

Compositions A through K, below illustrate quench media which are within the scope of the present invention:

Oil A (a) 100 parts of triester of trimethylolpropane and C C acids (avg. C35) (b) 0.3 part of phenothiazine (c) 3 .0 parts of dioctyl diphenylamine.

Oil C (a) 100 parts of tetraester of pentaerythritol and mixed C C acids (Hercoflex 600) (b) 1 part phenothi-azine (c) 6 parts dioctyl diphenylamine.

Oil D (a) 100 parts of tetraester of pentaerythritol with mixed C C acids (Hercoflex 600) (b) 1 part phenyl alpha naphthylamine.

Oil E (a) parts of monoester obtained by reacting C Guerbet alcohol with coconut fatty acids (b) 20 parts of a mineral lubricating oil having a viscosity of 80 S.S.U. at F. (c) 1 wt. percent phenyl beta naphthylamine.

Oil F (a) 50 parts of triester of trimethylol propane and palmitic acid (b) 50 parts of tetraester of pentaerythritol and mixed C1ZC15 acids.

Oil G (a) 100 parts of oil F (b) 0.5 part of phenothiazine (c) 4.8 parts of dioctyl diphenylamine Oil H (a) 100 parts of di(n-decyl) sebacate (b) 1 part phenyl alpha naphthylamine.

Oil I p (a) 30 parts of diester of 4,7-dithia-1,IO-decanedioic acid and j3-tert-octylmercaptoethanol (b) 60 parts of triester of trimethylol butane and 1,1-

10 384 hours) the Weight loss, acidity and the percentage in crease in viscosity at 210 F. are measured. Comparative tests are carried out on a commercially available hydrocarbon quenching oil (L) and on the same tetra-ester condimethylvalericacid 5 taining 1 wt. percent phenyl alphanaphthylamine. The (c) parts of a complex ester of the formula alcoholresults of such a test are shown in the following table:

TABLE I Duration Weight Viscosity of test loss, Acidity increase (hours) percent (mgKOH/g.) at 210 F. (percent) OilL 192 2 011 gelled. Tetraester 192 10 his 1% pheny1 a-naphthylamine 384 29 6 8 192. Tetraester 192 11 lus fi7gldi(octylphenyl)amine 384 11.5 1 6 40.

11 1% p1 1enothiazine 1 Still fluid.

acid-glycol-acid-alcohol wherein the alcohol is Z-ethyl It is clear from these results that the oxidation of a quench hexanol, the acid is brassylic acid and the glycol is oil prepared according to the present invention is much triethylene glycol. less than that of the other oils tested, as shown by the Oil] much smaller increase in viscosity and acidity values. (a) 100 parts of tetraester of pentaerythritol and mixed The Somewhat greater volallhty as Shown by.the Welght loss can be avoided by using, as the base 011, an ester C -C acids f C C f 1 h (b) 1 part phenothiazine (e.g. tetraester o 15 act s), 0 lower v0 at] ity suc (C) 6 parts dioctyl diphenylamine. as have been previously described.

Oil K EXAMPLE 3 (a) 15 parts of mineral oil having a viscosity of 90 S.S.U. The inventive oils of Example 2 are also subjected to at 100 F. oven tests in the presence or absence of oxygen, the (b) 20 parts of triester of 1,2,4-tricarboxy butane with former being carried out without any attempt to exclude n-decyl alcohol air and the latter being carried out in a system from which (c) 65 parts of diester of neo pentyl glycol and stearic air is largely excluded by purging the oil and test equipacid ment for 20 minutes with nitrogen at the beginning of the (d) 1 part phenyl alpha naphthylamine. 40 test. A little oxygen may, however, remain dissolved in EXAMPLE 2 To illustrate the beneficial effect of antioxidants, and especially the preferred antioxidant combination, the fol lowing procedure is followed:

the oil,v or enter the apparatus during sample changing. The results of such oventests are shown in the following table (Table II), together with comparative results for two commercially available hydrocarbon quenching oils (L and M).

TABLE II.TEST CONDITIONS phenyl alpha naphthylamine. di (0 ctylphenyl) amine. phenothiazine.

Ox air not excluded. Non-ox air excluded.

The previously described tetra-ester of pentaerythritol (known commercially as Hercoflex 600) is admixed with 6% of a commercially available di-(octyl phenyl) amine in which the octylphenyl group is a mixture of isomeric structures and further admixed with 1% phenothiazine, the percentages being by weight based on the weight of the tetraester. The quench oil thus obtained has a kinematic viscosity of 16 to 18 centistokes at C. (132 F.), 4.8 to 4.9 centistokes at 210 F., and about 0.8 centistokes at 210 C.

This quench oil is then subjected to oxidation tests in which 300 volumes of air per volume of oil per hour are bubbled through the oil at 185 C. for 192 hours and These results again show that the oxidative degradation of the quench oils prepared according to the present invention is much less than that of the other oils tested. The thermal stability of the oil prepared according to a preferred form of the present invention -is considerably better than that of the same tetra-ester containing 1% phenyl alpha naphthylamine, as shown by the much smaller increase in viscosity in the virtual absence of oxygen. Both are far superior to the two mineral oils in the presence of oxygen. Even better thermal stability can be achieved by using as the base oil for example a tetraester of carboxylic acid substituted at the alpha carbon atom by two alkyl groups, such as those previously de- 384 hours. At the end of each period (192 hours and scribed.

11 EXAMPLE 4 The quench oil of Examples 2 and 3 comprising Hercoflex 600 and 1% of phenyl ot-naphthylamine is used to quench steel gear wheels and other parts. The steel parts are heated to 830 C. and then plunged into a bath containing the quench oil, open to the atmosphere and maintained by thermostatic control at a temperature of 200 C. Over a period of 3 months, a total of 7.4 metric tons of steel parts are quenched in this manner, these parts having a total surface area of 185 sq. m. During this time the make-up of oil necessary to counteract the losses due to oil take-up by the parts and due to volatility losses is only 6.9 litres i.e. about 1 litre per metric ton or about 4 litres per 100 sq. m. of surface area of parts quenched. During this period also, the viscosity of the quench oil has only increased from 18 centistokes to 61 centistokes at 50 C.

Attention must be drawn to the conditions of the abovedescribed example, in which the quench oil of the invention was exposed to the atmosphere. In large scale continuous practice the bath would be enclosed and the oil exposed to an inert or reducing atmosphere, e.g. propane, or a mixture of carbon monoxide, hydrogen and nitrogen. Under such conditions, degradation of the quench oil would be very much less even than in the example (as shown by viscosity increase) and the quench oil would have an almost indefinite lifetime, requiring replacement in toto only at very long intervals, and only requiring make-up to replace the small losses due to take-up by the quenched parts and volatility losses, as noted above. This property of the quench oils of the invention makes for great economies in the quenching process after the initial investment costs.

Having described the present invention with a certain degree of particularity, it will be realized that numerous modifications and adaptations may be made within the spirit and scope of the invention as hereinafter claimed.

What is claimed is:

1. In a method of quenching useful in heat treatment of metals wherein a metal to be treated is heated to an elevated temperature and wherein said heated metal is then rapidly quenched in a quenching medium to efiect desired metallurgical changes in said metal, the improvement which comprises using as said quenching medium a liquid composition comprising a major proportion of synthetic carboxylic acid ester.

2. A method as defined in claim 1 wherein said ester comprises dibasic acid dies-ter.

3. A method as defined in claim 1 wherein said ester comprises full ester of neopentyl polyol.

4. A method as defined in claim 1 wherein said quenching medium has a viscosity of below about cs. at 210 F.

5. A method as defined in claim 1 wherein said ester comprises neopentyl polyol ester of C C acid.

6. A method as defined in claim 1 wherein said ester comprises tetraester of pentaerythritol and acid having an average chain length of at least 11.

7. A method as defined in claim 6 wherein said acid has an average chain length of from 14-18 carbon atoms.

8. A method as defined in claim 1 wherein said quenching medium also includes an oxidation inhibiting amount of antioxidant.

9. A method as defined in claim 8 wherein said antioxidant comprises 04 to 2 wt. percent phenothiazine and 4-6 wt. percent secondary aromatic amine.

10. A method as defined in claim 5 wherein said neopentylpolyol is selected from the group consisting of trimethylolpropane, pentaerythritol, and mixtures thereof.

11. A method as defined in claim 10 wherein said acid contains an average of at least 11 carbon atoms.

12. In a method of quenching useful in heat treatment of metals wherein a metal to be treated is heated to an elevated temperature and wherein said heated metal is then rapidly quenched in a quenching medium to efiect desired metallurgical changes in said metal the improvement which comprises using as said quenching medium a composition consisting essentially of tetraester of pentaerythritol and acid having an average chain length of from about 14-18 carbon atoms, from 0.5 to about 1 wt. percent phenothiazine, and from 4 to about 6 wt. percent of di(alkyl phenyl) amine in which the alkyl groups each contain from 4-12 carbon atoms.

13. In a method of quenching useful in heat treatment of metals wherein a metal to be treated is heated to an elevated temperature and wherein said heated metal is then rapidly quenched in a quenching medium to efiect desired metallurgical changes in said metal the improvement which comprises using as said quenching medium a composition consisting essentially of tetraester of pen taerythritol and neo acid having a chain length of at least 5 carbon atoms.

14. A method as defined in claim 13 wherein the quenching medium also contains from 0.5 to about 1 wt. percent phenothiazine, and from 4 to about 6 wt. percent di(alkylphenyl)amine in which each alkyl group contains from 4-12 carbon atoms.

References Cited by the Examiner UNITED STATES PATENTS 2,889,354 6/1959 Blake et a1 25257 X 2,922,763 1/ 1960 Tierney 25279 X 2,956,954 10/1960 Hoare et al. 25257 2,957,022 10/1960 Cohen 252-57 X 3,048,542 8/1962 Tierney et a1 25257 X 3,113,054 12/1963 Billett et al. 14829 3,148,147 9/1964 Bell et al. 252--79 X 3,159,510 12/ 1964 Rozalsky et al 14829 DAVID L. RECK, Primary Examiner.

C. N. LOVELL, Assistant Examiner. 

12. IN A METHOD OF QUENCHING USEFUL IN HEAT TREATMENT OF METALS WHEREIN A METAL TO BE TREATED IS HEATED TO AN ELEVATED TEMPERATURE AND WHEREIN SAID HEATED METAL IS THEN RAPIDLY QUENCHED IN A QUENCHING MEDIUM TO EFFECT DESIRED METALLURGICAL CHANGES IN SAID METAL THE IMPROVEMENT WHICH COMPRISES USING AS SAID QUENCHING MEDIUM A COMPOSITION CONSISTING ESSENTIALLY OF TETRAESTER OF PENTAERYTHRITOL AND ACID HAVING AN AVERAGE CHAIN LENGTH OF FROM ABOUT 14-18 CARBON ATOMS, FROM 0.5 TO ABOUT 1 WT. PERCENT PHENOTHIAZINE, AND FROM 4 TO ABOUT 6 WT. PERCENT OF DI(ALKYLPHENYL) AMINE IN WHICH THE ALKYL GROUPS EACH CONTAIN FROM 4-12 CARBON ATOMS. 